The present invention relates generally to machining, and, more specifically, to electrical erosion machining.
Modern gas turbine engines include multiple rows of rotor blades mounted to the perimeter of a supporting rotor disk. Compressor blades are used for pressurizing air in the compressor, and turbine blades are used for expanding combustion gases for powering the supporting disk.
A high pressure turbine powers the compressor through a drive shaft extending therebetween. And a low pressure turbine powers an upstream fan in a turbofan aircraft engine application. In marine and industrial applications, the low pressure turbine powers an external drive shaft for powering a marine propeller or an electrical generator, for example.
The gas turbine engine includes many stages of compressor and turbine blades. A multitude of such blades are therefore found therein. The blades in each stage or row are identical to each other, and typically vary in size from stage to stage.
Various manufacturing processes are available for efficiently manufacturing compressor and turbine blades, yet nevertheless the multitude of such blades requires a substantial expenditure of resources and time, which affect the rate of production and cost of the final engines.
Compressor blades are typically solid, superalloy metals initially manufactured from a respective preform. The preform is typically made from bar stock by an upsetting process that plastically deforms the metal to form a relatively large dovetail hub and integral, relatively thin airfoil stub. The preform includes excess material around its full perimeter so that the final configuration of the rotor blade may be suitably machined therefrom.
However, these preforms are made of superalloy metals, such as nickel based superalloys, which have enhanced strength for use in the gas turbine engine, which enhanced strength increases the difficulty of machining metal during the manufacturing process.
The airfoil of the blade must be precisely machined for achieving the generally concave pressure side thereof and the opposite, generally convex suction side that extend in span from root to tip and extend in chord between opposite leading and trailing edges.
The dovetail of the blade is also precisely machined to form a pair of opposite lobes or tangs having the typical dovetail shape for mounting the individual blades in corresponding dovetail slots in the perimeter of a supporting rotor disk. The dovetails may either be axial-entry supported in axial slots in the rim of the rotor disk, or may be circumferential-entry and supported in a circumferential dovetail slot extending around the rim of the rotor disk.
Accordingly, the airfoil and dovetail are configured differently from each other, yet nevertheless must have precise configurations relative to each other for being accurately supported to the perimeter of the rotor disk in efficiently performing their compression function on the air channeled through the compressor during operation, while withstanding the substantial aerodynamic and centrifugal loads experienced during operation.
In one conventional manufacturing process for compressor rotor blades being commercially used in the United States for many years, the compressor blades are formed in a series of processes. Firstly, the preform described above is fixtured around its airfoil stub and then the dovetail hub is precision ground to a rough form or dovetail having a generally H or I configuration with grooves on opposite sides thereof forming reference datum surfaces.
The preform is then fixtured around the rough dovetail in a conventional electrochemical machining (ECM) machine in which a pair of electrode tools or plates are translated together on opposite sides of the airfoil stub to electrochemically machine away material therefrom and form a precisely configured airfoil within the required small tolerances thereof expressed in a few mils.
Electrochemical machining is a conventional practice in which the preform and electrode plates are electrically powered as anode and cathode, respectively, and a liquid electrolyte is circulated in the gaps therebetween. ECM removes material from the airfoil stub as the opposite plates are translated inwardly toward each other to reach the final thickness and shape of the airfoil corresponding to the complementary profiles of the two electrode plates.
Since the airfoil is now machined to its final configuration and surface finish, it must be suitably protected during subsequent manufacturing processes by typically being encapsulated in a suitable soft metal matrix such as tin and bismuth.
The encapsulated airfoil may then be suitably fixtured again in a precision grinding machine so that the rough dovetail may be precision ground to an intermediate rough tang shape that closely encloses the desired configuration of the final dovetail. Precision grinding exerts substantial loads on the preform, which loads must be suitably supported in the corresponding fixtures therefor. Since the initial airfoil stub is oversized, a relatively simple fixture or clamp may be used for supporting the stub.
However, the machined airfoil has its final dimensions and surface finish, and simple fixturing thereof could lead to unacceptable damage to the airfoil, and therefore the airfoil is protected by encapsulation which encapsulation may then be simply fixtured in the grinding machine.
Following grinding of the finished tang, the final configuration of the dovetail may then be obtained in any conventional machining process, such as by additional precision machining of the individual dovetail tangs or lobes thereof.
When the dovetail is finally machined, the airfoil may then be unencapsulated by simply melting the soft metal matrix therefrom. The preform now includes finally finished airfoil and dovetail and integral platform therebetween, and then undergoes typical finishing processes such as trimming the length of the airfoil to the required tip height, for example.
The separate processes typically required in machining the preforms to the finally configured rotor blades typically require multiple machines and multiple operators, which also correspondingly increases the attendant costs.
In view of the large number of blades required in each stage of the turbine engine, the compressor blades are typically processed in relatively large batches. For example, eight of the preforms may typically undergo precision grinding at one time to form the rough dovetails. Forty of the preforms may undergo precision grinding around the perimeter of a supporting fixture to form the rough tangs thereof.
The ECM process is typically conducted on a single blade at a time since the electrode plates are form fit to complement the generally concave pressure side of each airfoil and the generally convex suction side of each airfoil which typically vary from root to tip and between leading and trailing edges.
Although the batch processing of the preforms permits simultaneous machining thereof for reducing processing time, processing time is nevertheless increased for the additional time needed to fixture the individual blades in the grinders. Additional time is also required for encapsulating each of the multitude of blades, and later unencapsulating those blades. And, the large batch processing of the preforms renders uneconomical the processing of fewer blades when required for lower production requirements.
Accordingly, it is desired to provide an improved process for electromachining rotor blades for reducing the complexity thereof and promoting efficient manufacture of small batches.